1. Field of the Invention
The present invention relates to a chrominance signal processing system such as an automatic phase control circuit (hereinafter to be referred to as an APC circuit) to obtain signals which are synchronous with the burst signal of an input carrier chrominance signal for use in a video tape recorder and the like.
2. Description of the Prior Art
Recently, there have been increasing demands to process the video signals including the chrominance signal to be used in a video tape recorder and the like in the form of a digital signal. In so doing, in order to obtain a clock which is synchronous with the burst signal, an APC circuit is used.
With reference to the drawings, description will be made on the conventional APC circuit hereunder. FIG. 1 is a block diagram to show the conventional APC circuit used for processing the chrominance signal in a digital signal form. In the figure, a video signal including the chrominance signal, inputted from an input terminal 1, is converted into a digital signal by an A/D (analog/digital) converter 2. This digital signal is outputted from an output terminal 3, and simultaneously inputted to a phase comparator 4 where its phase is compared with the output of a VCO (voltage controlled oscillator) 8.
Then, the output of the phase comparator 4 passes through a switch circuit 5 which is closed only for the burst period, and filtered by a low-pass filter 9 constituted by resistors R.sub.1, R.sub.2, a capacitor C and an operational amplifier 7 to control the VCO 8. The output of the VCO 8 is sent to the A/D converter 2 as a clock, and as a result, the input signal from the input terminal 1 is subjected to A/D conversion by a clock which is synchronous with the burst signal.
However, in the constitution such as above, there has been a defect in that, because the phase difference between the output of the VCO 8 and the burst signal is determined by the voltage of a voltage source 6 connected to the positive input terminal of the operational amplifier 7, the above phase difference fluctuates with the variation of the voltage of the voltage source 6. This is because of the fact that since the low-pass filter 9 filters the difference between the voltage of the voltage source 6 and the output of the phase comparator 4 which has passed through the switch circuit 5, unless the voltage to be outputted from the phase comparator 4 at the time when the phase difference is zero is equal to the voltage of the voltage source 6, feedback is not applied so that the phase difference between the two signals to be compared by the phase comparator 4, i.e., the phase difference between the burst signal and the output of the VCO 8, becomes zero.